Interesting deal. From the picture, I might be inclined to rotate the system 90deg to one of the end walls. If you do go in wall, those angled sides might give a better angle to the LP. Also, the angled wall above the mains might add some funny reflections.(maybe). What are your mains and center, and budget? Fwiw, you can get in walls from jbl, kef, Martin Logan etc too. This makes it easier to timbre match. While it's not critical to match, it does yield benefits as mentioned, and my recommendation is that IF you can, you should. I like to go to BB and mess with their heads. Lol
Most of us aren't fans of in wall speakers but, sometimes that's just what works. As far as hiding wires, I tuck mine under base trim as much as I can. There are cover strips and things available too. And if you go in wall, or on wall for surrounds, you'll likely be in the attic. Then you can use wall plates like these.
nice and clean.